The prostaglandin synthetase-dependent cooxidation of (±)-trans-7,8-dihydroxy-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene (BP-7,8-diol) was investigated using enzyme preparations derived from various mammalian tissues, including human lung. Incubation of this compound with arachidonic acid (AA) and ram seminal vesicle microsomes resulted in the formation of the benzo(a)pyrene (BP) tetrols 7,10/8,9 tetrol of BP and 7/8,9,10 tetrol of BP, which are formed upon solvolysis of (±)-trans-7β,8α-dihydroxy-9α, 1α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene. Microsomes derived from several target organs (rat lung and intestine, mouse skin, and guinea pig lung) and rabbit kidney also metabolized BP-7,8-diol to the 7,10/8,9 tetrol of BP and the 7/8,9,10 tetrol of BP in the presence of AA. Indomethacin inhibited AA-dependent oxidation in all tissues. The amounts of BP tetrols formed by the above tissues were comparable to that formed by ram seminal vesicles, even though these tissues possessed less than 1% of the prostaglandin synthetase activity of ram seminal vesicles. Using ram seminal vesicles as a model system, the optimal enzyme level was found to occur at very low levels of enzyme, actively suggesting that significant oxidation would occur in target organs which have a low level of prostaglandin synthetase. Also, all the tissues examined catalyzed the covalent binding of BP-7,8-diol to microsomal protein (6 to 13% of the total substrate metabolized), indicating the formation of reactive metabolite(s) during cooxidation.
Cytochrome P-450 monooxygenase- and prostaglandin synthetase-dependent BP-7,8-diol oxidation were compared using enzyme preparations derived from pulmonary tissues. In the presence of reduced nicotinamide adenine dinucleotide phosphate, rat lung microsomes metabolized BP-7,8-diol to BP tetrols at a rate approximately 10 times faster than in the presence of AA. Using guinea pig lung microsomes, however, BP tetrol formation was about equal when either of these cofactors were utilized. Human lung microsomal prostaglandin synthetase-dependent oxidation of BP-7,8-diol was between 52 and 67% of the cytochrome P-450-dependent rates. Using these pulmonary tissues, BP-7,8-diol was converted to the 7,10/8,9 tetrol of BP and the 7/8,9,10 tetrol of BP in the presence of AA. In the presence of reduced nicotinamide adenine dinucleotide phosphate, two additional BP tetrols were formed, the 7,9,10/8 tetrol of BP and the 7,9/8,10 tetrol of BP. These results suggest that prostaglandin synthetase may serve as an alternative activating enzyme to cytochrome P-450 monooxygenases in the conversion of BP-7,8-diol to (±)-trans-7β,8α-dihydroxy-9α, 10α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene.